1. Field of Invention
The invention relates generally to photobioreactors, particularly floating photobioreactors, and processes to operate and use the photobioreactors for the production of biomass, and for the treatment of gases, such as flue gases and/or carbon dioxide-enriched gases.
2. Discussion of the Related Art
The global market for fossil feedstocks for transportation fuels is increasingly characterized by tight supplies and threats resulting from geopolitical instability leading to increasing calls for clean, reliable, domestic sources of liquid fuel. Production of ethanol derived from corn is rapidly increasing to meet this demand, urged on by government incentives, but the net energy gain of corn ethanol is modest at best, and the feedstock is subject to competitive demand from food and feed markets. Conversion of soybean oil to biodiesel is also increasing, but production capacity is likewise constrained by competing demand. Cellulosic ethanol derived from switchgrass and crop residues is an area of active research, but the process is not economical at present, and significant progress is needed to make it competitive. As the market for biofuels grows, feedstock production increasingly competes with food and livestock interests for a limited amount of good agricultural land and clean irrigation water, and so a need is developing for biofuel feedstocks that do not require or require less of these resources.
Evidence is increasing that significant global warming is occurring as a result of human activities, chiefly the release of massive quantities of carbon dioxide from the burning of coal, oil, and other fossil fuels. These fuels are used not only for transportation, but also for electricity generation, industrial processes, and heating. Concerns relating to potential political, economic, and agricultural disruptions resulting from climate change have led to modest initial efforts to reduce global carbon dioxide emissions, such as the Kyoto Protocol. The prospect of further carbon dioxide control measures such as possible emission caps and carbon taxes have also spurred interest in renewable, “carbon-neutral” biofuels.
Because the burning of fossil fuels provides the majority of the energy used by humans, a reduction in fossil fuel use could have a significant economic impact unless alternative energy sources were available to offset the reductions. Another possibility is separation of carbon dioxide from fossil fuel exhaust gas streams, followed by sequestration or recycling.
One approach that addresses the problem of carbon dioxide emissions is the use of flue gas as a carbon source to accelerate the growth of photosynthetic algae. Algae are the fastest growing plants on earth and one of nature's simplest microorganisms, and they are also one of the most efficient converters of carbon dioxide and solar energy to biomass. Certain previous efforts at using algae for fuel production or carbon dioxide mitigation have employed different approaches but because of various problems, addressed by certain embodiments of the present invention, they have encountered various difficulties that have limited their technical and commercial success.